1. Field of the Invention
The present invention relates generally to OFDM based wireless receivers, and more particularly to automatic gain control (AGC) and DC offset cancellation in a wireless receiver having a differential path.
2. Prior Art
Receivers are necessary components of communication links, and are used, for example, in two-way cellular phone communications and wireless local area networks. A simplified block diagram of a typical prior-art wireless receiver 100 having an inphase or “I” path and a quadrature or “Q” path is shown in FIG. 1. The “I” and “Q” paths are typical of a direct-conversion receiver which employ the two channels, commonly referred to as the “I” and “Q” paths. In such a receiver, a signal from an antenna is fed to low-noise amplifier (LNA) 110. LNA 110 is capable of a low or high gain, controlled through a baseband chip (not shown). LNA 110 is needed for the purpose of amplifying weak signals without introducing much noise. Furthermore, the gain of LNA 110 is not continuous, as it has only two gain settings, being externally controlled. An AGC circuit is connected from the output of a baseband amplifier (BBA), for example BBA 140-I, and its output connected to the amplification control signal of BBA 140-I. LNA 110 feeds a mixer 120, for example mixer 120-I, which mixes down the received high-frequency signal to the baseband (including 0 Hz), by effectively multiplying the received and then amplified signal with a local-oscillator (LO) signal produced by an oscillator (not shown), for example LO “I”, in the receiver. The undesirable signals at very high frequencies produced by this process are filtered out by using a band pass filter BBF, for example BBF 130-I. The filtered signal is then amplified by a baseband amplifier (BBA), for example BBA 140-I, and is output as VI of the “I” channel. The gain of BBA 140-I is made variable through AGC action; the gain is made large when the received signal is small, and small when the received signal is large. The objective of this operation is to keep the output signal to a well-defined power so that it can be encoded by an analog-to-digital converter or otherwise used without undue distortion and noise. There is a symmetrical channel to provide the “Q” channel of the wireless receiver, the I and Q signals to the mixers being 90° out of phase.
A wireless receiver, operating for example in accordance with the IEEE 802.11a standard, uses orthogonal frequency division multiplexing (OFDM) with a preamble sequence 200 shown in FIG. 2. The preamble field is composed of ten repetitions of a “short training sequence” 210, used for AGC convergence, diversity selection, timing acquisition and DC offset cancellation in the receiver. The preamble field is further composed of two repetitions of a “long training sequence” 220, used for channel estimation and fine frequency acquisition, preceded by a guard interval 230. A short OFDM training symbol consists of 12 sub-carriers (±4, ±8, ±12, ±16, ±20 and ±24 with 312.5 KHz of spacing, for 802.11a and ±2, ±6, ±10, ±14, ±18 and ±22 with 312.5 KHz of spacing, for Hiperlan2). As there is no DC content in the spectral range but there is a DC offset error, this leads to an additional error in the AGC functionality. Specific operation of an AGC loop is well-known in the art and therefore a detailed analysis is not provided herein. Prior art solutions for AGC using frequency offset estimation methods first perform a coarse measure using the last few, usually three, short symbols of the first portion of the preamble. After that a second fine measure is performed, using the second portion of the preamble. The number of symbols used for coarse measurement is a compromise between the need to achieve maximum precision which requires a large number of symbols and the phase ambiguity where a larger estimation range must be used and therefore a small number of symbols. The resultant compromise is as described above, leading to a less accurate estimation of frequency offset, impacting the capability of the AGC.
In view of the limitations of prior art solution, it would be advantageous to provide means for an effective AGC for system 100, preferably less dependent on the number of symbols. It would be further advantageous if such AGC means would further control DC offset cancellation means without interfering with the AGC operation.